| Term | Definition |
|---|
| astronomy | The study of the moon, stars, and other objects in space. |
| rotation | The spinning motion of a planet about it's axis. |
| revolution | The movement of an object around another object. |
| satellite | Any object that travels around another object in space. |
| inertia | The tendency of a moving objectto continue in a sraight line or a stationary object to remain in place. |
| gravity | The force that pulls objects towards Earth. |
| nuclear fusion | The process in which nuclei combine into larger nuclei, forming heavierelements and raising energy. |
| terrestrial planets | The name given to the four inner plantets/ Mercury, Venus, Earth, Mars. |
| retrograde rotation | The spinning motion of a planet from east to west, oppsite to the direction of rotation of most plantets and moons. |
| gas giants | The name given tp the first outer plantets/ Jupiter, Saturn, Uranus, Neptune. |
| comet | A ball of ice and dust whose orbit is a long, narrow ellipse. |
| asteroid | Objcts revolving around the sun that are too small and numerous to be considered plantets. |
| meteor | A streak of light in the sky produced by the burning of a meteorid in Earth's atmosphere. |
| meteorite | A meteoroid that has hit the Earth's surface. |
| galaxy | A giant structure that contains hundreds of billions of stars. |
| universe | All of space and every thing in it. |
| astronomical unit | The average distance from Earth to the sun- about 150 million kilometers. |
| light- year | The distance that light travels in one year. |
| apparent magnitude | The brightness of a star as seen from Earth. |
| absolute magnitude | The brightness from a star if it were a standard distance from Earth. |
| protostar | a contracting cloud of gas and dust; the earlliest stage of a star's life. |
| nebula | A large amount of gas and dust in space, spread out in an immense volume. |
| white dwarf | The remaining hot core of a star after its outer layers have expanded and drifted out into space. |
| neutron | Small uncharged particles in the nucleus of an atom. |
| black hole | The remains of an exremely massive star pulled into a small volume by the force of gravity. |
| spiral galaxy | A galaxy whose arms curve outward in a pinwheel pattern. |
| elliptical galaxy | A galaxy shaped like a flattened ball, containing only old stars. |
| irregular galaxy | A galaxy that goes not have the a regular shape. |
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